Some aircraft engines include a mechanically driven accessory gearbox that drives accessory systems such as fuel pumps, scavenge pumps, electrical generators, hydraulic pumps, etc. An accessory gearbox may include a number of gears that convert the rotation of an engine shaft into different rotation speeds that are suitable for various accessory systems. Each of the accessory systems may be driven by a rotating drive shaft.
If one of the accessory systems experiences a mechanical abnormality, continued rotation of the accessory system may damage the engine or the accessory gearbox. For instance, if an accessory system experiences a loss of lubricating or cooling oil, continued rotation of the accessory system may cause the accessory system to lockup and/or overheat, which could potentially damage the engine or the accessory gearbox.
The drive shafts that drive some accessory systems are designed to break and disconnect the accessory system from the accessory gearbox when a mechanical abnormality occurs. For example, a drive shaft may, by design, include a region of reduced thickness where structural failure may occur under excessive loading. This region is commonly referred to as a shear neck. In operation, if the accessory system that is driven by the drive shaft levies excessive torque on the drive shaft, the shear neck may fail, thereby disconnecting the accessory system from the drive shaft. If, however, the shear neck is sized too large, the accessory system might not be disconnected fast enough and the malfunctioning accessory system could damage the engine or accessory gearbox. On the other hand, if the shear neck is sized too small, the accessory system might be disconnected too often.